A capacitive humidity sensor, disclosed in U.S. Pat. No. 6,580,600 corresponding to JP-A-2002-243690, detects a change in relative humidity in the atmosphere using a capacitance change.
FIG. 5A is a plan view of a capacitive humidity sensor 100 similar to the sensor disclosed in U.S. Pat. No. 6,580,600. The sensor 100 includes a sensor element 10 and a reference element 90. The sensor element 10 has a pair of comb electrodes 10a, 10b, and the reference element 90 has a pair of comb electrodes 90a, 90b. The electrodes 10a, 10b and the electrodes 90a, 90b are disposed on the same surface of a semiconductor substrate 1.
FIG. 5B shows a relationship between relative humidity and capacitances of the sensor element 10 and the reference element 90. A humidity sensitive layer 4 is formed on the electrodes 10a, 10b of the sensor element 10. In contrast, the humidity sensitive layer 4 is not formed on the electrodes 90a, 90b of the reference element 90. As shown in FIG. 5B, therefore, capacitance Cv of the sensor element 10 changes in accordance with a change in relative humidity, whereas capacitance Cr of the reference element 90 is constant relative to a change in relative humidity.
The sensor element 10 is connected in series with the reference element 90. In FIG. 5A, V12 represents a voltage between the electrodes 10a, 10b of the sensor element 10. V23 represents a voltage between the electrodes 90a, 90b of the reference element 90. A change of the Cv is reflected in a variation of the ratio of V23 to V12, because the Cv is represented by the following equation:Cv=(V23/V12)Cr
In the sensor 100, the capacitance Cv changed in accordance with a change in relative humidity is compared with the capacitance Cr, and then the variation of the ratio of V23 to V12 is detected. Relative humidity is calculated based on the variation of the ratio of V23 to V12 using a sensor signal processing circuit, which is formed on the substrate 1. Thus, the sensor 100 detects relative humidity in the atmosphere. The signal processing circuit is integrated with the substrate 1 so that the sensor 100 has a small size and can be manufactured at low cost.
As regards the sensor 100, however, it has been shown that the electrodes 90a, 90b of the reference element 90 were damaged and a value of the capacitance Cr of the reference element 90 was changed, after the sensor 100 was left for a long time under high-temperature and high-humidity condition. The electrodes 90a, 90b may be protected from damage by applying a protective material such as gel to surfaces of the electrodes 90a, 90b. In the case, however, the protective material may spread over the sensor element 10 adjacent to the reference element 90 because of the small size of the sensor 100. As a result, responsivity of the sensor element 10 is reduced. Further, manufacturing cost increases because of an additional process for applying the protective material to the electrodes 90a, 90b. 